If I may,
I emailed Linux support at the end of November (2019) and Sergii kindly answered me back. I shared some of the worries expressed in this thread and other private conversations with fellow professionals outside this forums and, in a nutshell, the situation is, as I was told, this:
A lot has changed in the code. And he's working non-stop to catch up for the Mac version to be ready. Once that port is done, Linux is next. He assumed that around the time 3DCoat 2020.xx is out we'll find the builds popping out. Also, from then, serial keys would be for all systems.
So there you go. It's going to take some time, but apparently it's not an abandoned project. I don't worry particularly that the Mac version comes first: the port from OSX to Linux presumably has to be relatively straight forward.
The perceived opacity around this facts seems a bit unnecessary to me because it's perfectly understandable that sometimes things take time in order to move forward. The team shared most of this information here and there but an easy to find official post in a thread would do wonders for current and potential customers' confidence.
All in all, I hope they can pull it out in the not so distant future. In the end it may very well be for the best.
Meanwhile, since WINE is not an option here, I hardly ever use 3DCoat. But the old build still works and it's being used occasionally. Not every job requires the new tools. It's the bugs and limitations long time gone in more current builds that frustrate me. Substance on the other side is fantastic in Linux, and in an Ubuntu (or derivative) it's not at all difficult to set up (I can help if someone needs me to.)
I hope I didn't add to the confusion. Being a professional, knowing where we are is important to me and I got what seemed an honest response. I hope when the time comes I can still make use of 3DCoat in my workflow, I really liked this software.